Method of estimating the position of a user device using radio beacons and radio beacons adapted to facilitate the methods of the invention

ABSTRACT

Disclosed is the estimation of the position of a user device ( 18 ) using radio beacons (B 1 -B 4 ) which are operable at a plurality of transmit power levels. The radio beacons (B 1 -B 4 ) may transmit their position and current transmit power level. Estimates of distance between a user device and a radio beacon can take into account both received signal strength and the current transmit power level of the radio beacon. The position of the user device ( 18 ) can be estimated taking into account whether a radio beacon (B 1 -B 4 )) can be detected at a given transmit power level. The transmit power level of the radio beacon may vary according to a cycle. The transmit power level of the radio beacon may be changed to facilitate positioning, for example in response to a signal from a user device. A radio beacon (B 1 -B 4 ) changes transmit power level in a cycle. The radio beacon can reduce transmit power level responsive to a signal. The radio beacon (B 1 -B 4 ) may be compatible with Bluetooth short range wireless connectivity standard core specification version 4.0.

This application is the U.S. national phase of International ApplicationNo. PCT/GB2012/052555 filed 15 Oct. 2012 which designated the U.S. andclaims priority to GB Patent Application No. 1117723.5 filed 13 Oct.2011 and claims the benefit of U.S. Provisional Application No.61/649,977 filed 22 May 2012, the entire contents of each of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The invention concerns a method of estimating the position of a userdevice using radio beacons which have a plurality of discrete transmitpower levels, and radio beacons adapted to facilitate estimates of theposition of a user device.

BACKGROUND TO THE INVENTION

In recent years there has been considerable interest in determining theposition of user devices, such as cellular telephones, portableelectronic computers and other portable personal user devices. Systemsbased on the use of global satellite navigation systems (e.g. GPS) areuseful out of doors but are of limited use within buildings and haveonly limited positioning accuracy. Accordingly, it is desirable todetermine the position of user devices using alternative technologies,particularly those which are useful indoors, either as an alternative toor in combination with global satellite navigation systems to obtainmore accurate measurements of position.

It is known to determine position by detecting radio beacons whichtransmit radio signals over relatively low distances, for example,connectable radio beacons which allows access to a LAN andnon-connectable radio beacons which simply transmit data, such as theiridentifier. Using a database of the position of such radio beacons it ispossible to determine position using techniques such as measuringreceived signal strength and triangulation.

The invention aims to provide improvements to these technologies, toenable more accurate measurements of position to be made, or to enablemeasurements to be made more simply, for example, some embodimentsenable an estimate of position without a requirement to measure receivedsignal strength. The invention also provides radio beacons whichfacilitate the methods of the invention.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof estimating the position of a user device comprising a radio receiver,the method comprising detecting one or more radio beacons operable atany of a plurality of discrete transmit power levels, and calculating anestimate of the position of the user device taking into account transmitpower data concerning the transmit power level of the one or more saidradio beacons at the time when the respective radio beacon is detectedand beacon position data concerning the position of the one or more saidradio beacons.

Thus, an estimate of the position of the user device can be obtainedusing the acquired knowledge of the position of some or all of thedetected radio beacons and data concerning the range at which the userdevice would typically be detectable. For example, if a single radiobeacon can be detected which is operating at a transmit power level atwhich it would typically be detectable at a range of 5 m, then theposition of the user device can be estimated as being within 5 m of theposition of the radio beacon. If two radio beacons can be detected, oneof which is operating at a transmit power level at which it wouldtypically be detectable at a range of 5 m, the other of which isoperating at a transmit power level at which it would typically bedetectable at a range of 10 m, the position of the user device can beestimated as within the locus of points 5 m from the first radio beaconand 10 m from the second radio beacon.

The one or more radio beacons may be Bluetooth beacons. The radioreceiver may be a Bluetooth receiver.

The beacon position data may be received from a database of the positionof radio beacons. However, the beacon position data may be received fromthe radio beacons, for example, some or all of the radio beacons maytransmit their position as part of the radio signals which theytransmit.

Some or all of the transmit power data may be received from a databaseof transmit power levels associated with specific radio beacons. Suchdata may comprise data indicative of the transmit power level whichspecific radio beacons will adopt at specific times.

Preferably, some or all of the transmit power data is received from theradio beacons. For example, the radio beacons may transmit data relatedto transmit power level to the user device as part of the radio signals(e.g. Bluetooth signals) which they transmit.

The transmit power data could be data specifying the transmit powerlevel (e.g. transmit power level in dBmW (power ratio of the transmitpower level to 1 mW express in decibels), or other appropriate units, ora label representative of a respective transmit power level). The datarelated to transmit power level could be expressed in a different way,for example, it could in principle be data specifying the range overwhich the radio beacon could be expected to be detectable (for example,a distance in meters or other units, or a label representative of arespective range over which a radio beacon could be expected to bedetectable).

The user device is typically a mobile personal user device, such as acellular telephone, personal digital assistant or laptop computer. Themethod of the invention requires the user device to have a radioreceiver (typically a radio transceiver) suitable for receiving signalsfrom the radio beacons, for example, WiFi or Bluetooth short rangewireless connectivity standard core specification version 4.0 compatiblelow range radio transceivers.

Typically the radio receiver is configured to receive signals complyingwith a short range (i.e. typically a range of less than 500 m, moretypically a range of less than 200 m, even more typically a range ofless than 100 m) wireless connectivity standard, such as Wi-Fi orBluetooth. Typically the one or more radio beacons are configured totransmit signals complying with a short range (i.e. typically a range ofless than 500 m, more typically a range of less than 200 m, even moretypically a range of less than 100 m) wireless connectivity standard,such as Wi-Fi or Bluetooth. In one embodiment, the one or more radiobeacons may be connectable radio beacons, such as Wi-Fi access points,and the radio receiver is configured to receive signals form theconnectable radio beacons. More typically, the one or more radio beaconsare Bluetooth beacons and the radio receiver is a Bluetooth receiver.That is, the one or more radio beacons are configured to transmitsignals complying with one or more of the Bluetooth short range wirelessconnectivity standards and the radio receiver is configured to receivesignals complying with that or those Bluetooth short range wirelessconnectivity standards. Some or all of the radio beacons may be, forexample, radio beacons compatible with the Bluetooth short rangewireless connectivity standard core specification version 4.0 or later,and the radio receiver may also be compatible with the Bluetooth shortrange wireless connectivity standard core specification 4.0 or later.Thus, preferably the radio beacons are configured to transmit signalscomplying with the Bluetooth wireless connectivity standard version 4.0or later and the radio receiver is preferably configured to receivesignals complying with the Bluetooth wireless connectivity standardversion 4.0 or later. The Bluetooth short range wireless connectivitystandard core specification version 4.0 is advantageous at is allowstransmission at any of a plurality of discrete transmit power levels.Bluetooth is a trade mark of the Bluetooth special interest group. TheBluetooth short range wireless connectivity standard core specificationis published from time to time at the domain www.bluetooth.org.

By the range at which the user device would typically be detectable werefer to the distance from the radio beacon at which it would typicallybe possible for a compatible radio receiving device to receive digitaldata reliably from (and in the case of a connectable radio beacon, totransmit digital data reliably to) the radio beacon without unusualcircumstances or structures between the radio beacon and the userdevice. In practice there will be some variation between user devices ordepending on the precise configuration of the environment around theradio beacon.

The methods of the invention are advantageous in that they are usefulindoors where positioning systems such as global satellite navigationssystems may not be available. Thus, the method may be a method ofestimating the position of a user device indoors (although it typicallyalso functions out doors). The method may be a method of estimating theposition of a user device where global satellite navigation signalscannot be received by the user device (although it typically alsofunctions where global satellite navigation signal can be received bythe user device).

It may be that, in at least some circumstances, the method does not takeinto account any measure of the strength of the signals received fromthe one or more radio beacons beyond whether or not the radio beaconscan be detected. It may be that, in at least some circumstances, themethod does not take into account any measure of the time of flight ofthe signals received from the one or more radio beacons. It may be thatin at least some circumstances (and in some embodiments, always), thedata received from the one or more radio beacons which is taken intoaccount is entirely digital data received from the one or more radiobeacons (e.g. an identifier of the radio beacon, data concerning theposition of the radio beacon etc.). Thus, the method comprises takinginto account whether radio beacons can be detected and, although it canin some embodiments be assisted by such data, does not rely on analoguemeasurements of signal strength or signal propagation time.

The step of detecting one or more radio beacons and calculating anestimate of the position of the user device taking into account transmitpower data concerning the transmit power level of the one or more saidradio beacons at the time when the respective radio beacon is detected,is typically repeated, e.g. periodically.

It may be that the method takes into account that one or more radiobeacons can not be detected at a given point in time. The method maytherefore comprise deducing that the user device is not within a locusat which a specific radio beacon of known position could be detected.This may be used for example in the situation where two radio beaconscan be detected to determine in which of two discrete loci on eitherside of a line joining the two radio beacons, the user device islocated.

Preferably, the method may take into account the transmit power level ofa radio beacon at a given time at which that beacon cannot be detected.The method may comprise taking into account that a radio beacon can bedetected at a time when it is outputting signals at a first transmitpower level and cannot be detected at a time when it is outputtingsignals at a second lower transmit power level. In the circumstance itcan be inferred that the user device is within an annulus centered onthe radio beacon, having an inner radius equal to the distance at whichsignals at the second lower transmit power level could typically bedetected and an outer radius equal to the distance at which signals atthe first transmit power level could typically be detected. Thus, ameasurement of received signal strength is not essential. It is eitherthe case that there is sufficient received signal strength for the radiobeacon to be detected and for digital data to be received from it, orthere is not. Nevertheless, it may be that calculating an estimate ofthe position of the user device takes into account measurements of thestrength of signals received from at least one of the one or moredetected radio beacons.

It may be that for some or all of the radio beacons, the respectiveradio beacon automatically switches between at least some of theplurality of discrete transmit power levels (typically periodically),for example it may change between discrete transmit power levels in acycle. The method may comprise controlling (e.g. programming orinstructing) some or all of the radio beacons to automatically switchbetween at least some of the plurality of discrete transmit power levelsaccording to a program, for example they may be controlled to changeperiodically between discrete transmit power levels in a cycle. A cyclemay be a cycle in which the transmit power level decreases monotonicallyfrom a highest level (which need not be the maximum transmit power levelof which the respective radio beacon is capable) through at least oneintermediate level to a lowest level before returning to the saidhighest level.

Therefore the method may comprise determining that a radio beacon couldbe detected when it was transmitting signals at a first transmit powerlevel but not when it was transmitting signals after changing to asecond lower transmit power level. The method may then comprisedetermining that the user device is located at a distance from the radiobeacon less than the distance at which signals at the first transmitpower level could typically be detected and greater than the distance atwhich signals at the second transmit power level could typically bedetected.

In a preferred embodiment, the method comprises the step of causing aradio beacon to change its transmit power level (by transmit power levelwe refer to a power level at which data can be transmitted, i.e. anon-zero power level at which the beacon can transmit data signals) tofacilitate the estimation of the position of the user device. The userdevice, or a positioning controller, may be programmed to generate asignal to cause a radio beacon to change its transmit power level tofacilitate the estimation of the position of a user device. Typically,the user device, or a positioning controller, generates a signal tocause a radio beacon to reduce its transmit power level from a firstdiscrete transmit power level to a second lower discrete transmit powerlevel. Typically the signal causes the radio beacon to reduce thetransmit power level at which it transmits directly. That is, the signalpreferably causes the radio beacon to reduce the transmit power level atwhich it transmits without ever changing (e.g. cycling) the transmitpower level through any higher power levels before reducing the transmitpower level. Preferably, the signal causes the radio beacon to reducethe transmit power level at which it transmits within 1 second ofreceiving the received signal, more preferably within 0.5 seconds ofreceiving the received signal, and even more preferably within 0.25seconds of receiving the received signal. In each case, the reductionfrom the first discrete transmit power level to a second discretetransmit power level typically takes place unless the first discretetransmit power level is the lowest discrete transmit power level of aplurality of discrete transmit power levels at which the radio beacon isoperable. If this (i.e. reducing the transmit power level of the radiobeacon) causes the user device to no longer be able to detect the radiobeacon then this enables a calculation to be made that the user deviceis at a distance from the radio beacon less than the distance at whichsignals at the first transmit power level could typically be detectedand greater than the distance at which signals at the second transmitpower level could typically be detected. If the radio beacon can stillbe detected by the user device, and if the radio beacon has a stilllower discrete power level, the method may comprise the user device or apositioning controller again causing the radio beacon to reduce itstransmit power level.

By enabling the user device or a positioning controller to change, andin particular to reduce, the transmit power level of the radio beacon,calculations taking into account whether or not the radio beacon can bedetected at given transmit power levels can be carried out more quickly.The transmit power level of the radio beacon may, however, be changedfor other purposes, for example it may be increased to obtain a strongersignal meaning that measurement of distance to the radio beacon (forexample based on received signal strength) will be more accurate (due toimproved signal to noise ratio). Any such increase may be temporary toenable the radio beacon to return to a lower transmit power level toreduce power consumption. It may be that such an increase in transmitpower level may be to a transmit power level above the maximum transmitpower level which the radio beacon adopts in a default mode, e.g. whenno user device is connected to it.

The user device may send signals to the radio beacon to change (e.g.reduce) its transmit power level directly. The user device may sendsignals to the radio beacon to change (e.g. reduce) its transmit powerlevel indirectly, for example, through a network, such as a cellularcommunications network, the internet etc. This process may be controlledby a positioning controller with which the user device is in electroniccommunication. The positioning controller may generate the signals toradio beacons to change (e.g. reduce) their transmit power level. Thepositioning controller may, for example, be a remote service inelectronic communication with the radio beacons. The positioningcontroller may be located proximate a plurality of radio beacons (forexample, in the same room or in the same building) and in communicationwith the plurality of radio beacons by wired or wireless connections.The positioning controller may be distributed, for example, betweenradio beacons. The positioning controller may be integrated into a radiobeacon which may in turn control one or more other radio beacons. Thepositioning controller may be a radio beacon controller. The positioningcontroller may comprise a Bluetooth interface for communicating directlywith the radio beacons and/or the user device. More typically, thepositioning controller may be in indirect electronic communication withthe radio beacons and/or the user device. For example, the positioningcontroller may be in electronic communication with or be a functionalmodule of a (typically remote) server which is in turn in electroniccommunication with the radio beacons and/or the user device. Saidelectronic communication may for example be over the internet andinclude wired/optical fibre/wireless connections or a mixture thereof.In an alternative example, the positioning controller may be inelectronic communication with the radio beacons and/or the user devicevia a longer range (i.e. longer range than Bluetooth) peer to peerconnection (e.g. over the Internet and/or a wireless connection such asWi-Fi or Wi-Max).

An advantage of embodiments in which a positioning controller generatesthe signals to radio beacons to reduce their transmit power level isthat, after a radio beacon has reduced its transmit power level it mayno longer be detectable by a user device. If the positioning controllerremains in electronic communication with the user device, then eitherthe user device or positioning controller can determine that the userdevice cannot detect the radio beacon at its reduced transmit powerlevel. It may be that the radio beacon transmits a signal representativeof the transmit power level to which it is about to change before itchanges. This is useful whether the radio beacon is increasing ordecreasing its transmit power level, but can be especially useful whenthe radio beacon is about to reduce its transmit power level. It isespecially useful in embodiments where the radio beacon transmits itspower level and the user device uses the power level informationtransmitted by the radio beacon to determine the transmit power level ofthe radio beacon. In some embodiments it will be sufficient that thesignal indicates simply that the transmit power level is about todecrease without it being essential to indicate the reduced transmitpower level.

Thus, it may be that the radio beacon, in response to receipt of a saidsignal causing the radio beacon to change (typically reduce) itstransmit power level, but before the radio beacon changes (typicallyreduces) its transmit power level, transmits a signal which isindicative that it is about to change its power level and/or indicativeof the transmit power level to which it is about to change.

It can be advantageous that the said signal is transmitted responsive toreceipt of a single to change transmit power level in that that providesa user device (or positioning controller where applicable) with ahandshake confirming that the radio beacon is changing its power levelresponsive to the signal which was transmitted to the radio beacon andnot for some other reason or due to potentially conflicting instructionsfrom another user device.

The transmitted signal may be transmitted to the user device. Thetransmitted signal may be transmitted to the positioning controller,where present. In some embodiments, the positioning controller transmitsa signal indicative that a radio beacon is about to change, is changing,or has just changed (e.g. reduced) its transmit power level and/or thepower level to which the transmit power level of the radio beacon isabout to change, is changing to, or has just changed.

The calculation of an estimate of the position of the user device may bemade by the user device, or remotely from the user device, for exampleby a server in electronic communication with the user device (e.g. overa cellular communication network, over the internet etc.), or a saidpositioning controller.

The method may comprise receiving a measure of an environmental propertywhich affects distance measurement with the radio (e.g. Bluetooth)signal received from the radio (e.g. Bluetooth) beacon, for example ameasurement of ambient temperature or atmospheric property, or anidentifier of a position calculation algorithm to use for positionmeasurement (for example, an identifier of or a parameter for anenvironmental model to be used for position calculation). The measure ofan environmental property may be measured by a sensor in the radiobeacon. This received data can be used to improve measurements such asthose based on received signal strength or time of flight measurements.

The invention also extends in a second aspect to a radio beacon (e.g. aBluetooth beacon) operable to transmit data using radio (e.g. Bluetooth)signals at any of a plurality of discrete transmit power levels, whereinthe radio beacon is programmed to change the transmit power level atwhich it transmits to facilitate estimation of the position of a userdevice.

By transmit power level we refer to a power level at which data can betransmitted, i.e. a non-zero power level at which the beacon cantransmit data signals.

The radio beacon may be configured to automatically switch between atleast some of the plurality of discrete transmit power levels (typicallyperiodically), for example it may change between discrete transmit powerlevels in a cycle. The radio beacon may be operable to automaticallyswitch between at least some of the plurality of discrete transmit powerlevels according to a program, for example to change periodicallybetween discrete transmit power levels in a cycle. A cycle may be acycle in which the transmit power level decreases monotonically from ahighest level (which need not be the maximum transmit power level ofwhich the respective radio beacon is capable) through at least oneintermediate level to a lowest level before returning to the saidhighest level.

The radio beacon may be configured (e.g. programmed) to change thetransmit power level at which it transmits responsive to a receivedsignal (typically to facilitate estimation of the position of a device,typically but not necessarily the device which transmitted the receivedsignal). The received signal may be a radio (e.g. Bluetooth) signalreceived from a user device which is receiving data from the radiobeacon. The received signal may be received from a controller, such as apositioning controller, for example using a wired or wireless network.Typically, the radio beacon changes the transmit power level at which ittransmits responsive to a received signal by reducing the transmit powerlevel at which it transmits (where possible). Preferably, the radiobeacon is configured (e.g. programmed) to reduce the transmit powerlevel at which it transmits directly in response to a received signal.That is, the radio beacon is configured (e.g. programmed) to reduce thetransmit power level at which it transmits in response to a receivedsignal without cycling the transmit power level through any higher powerlevels before reducing the transmit power level. Preferably, the radiobeacon is configured (e.g. programmed) to reduce the transmit powerlevel at which it transmits responsive to a received signal within 1second of receiving the received signal, more preferably within 0.5seconds of receiving the received signal, and even more preferablywithin 0.25 seconds of receiving the received signal. In each case, thereduction from the first discrete transmit power level to a seconddiscrete transmit power level typically takes place unless the firstdiscrete transmit power level is the lowest discrete transmit powerlevel of a plurality of discrete transmit power levels at which theradio beacon is operable. The radio beacon may then automaticallyincrease the transmit power level again after a predetermined period oftime.

It may be that the radio beacon is configured to transmit a signalindicative that it is about to change (e.g. reduce) transmit power leveland/or representative of the transmit power level to which it is aboutto change, before it changes. It may be that the radio beacon inresponse to receipt of a said signal causing the radio beacon to change(typically reduce) its transmit power level, but before the radio beaconchanges (typically reduces) its transmit power level, transmits a signalwhich is indicative that it is about to change its power level and/orindicative of the transmit power level to which it is about to change.

The radio beacon may be battery powered. The radio beacon typicallytransmits an identifier (e.g. MAC ID) of the radio beacon in itstransmitted radio (e.g. Bluetooth) signal. The radio beacon may transmitits position in its transmitted radio signal. The radio beacon maytransmit data concerning its current transmit power level in itstransmitted radio signal.

The radio beacon may transmit data to facilitate accurate distancemeasurement, for example, a measure of a property of the environmentadjacent the radio beacon, or an identifier or parameter of an algorithmfor use in position determination (such as an environmental model). Theradio beacon may comprise a sensor to measure a property of theenvironment around the radio beacon, for example air pressure ortemperature, and may be configured to transmit a measurement of thatproperty in its transmitted radio (e.g. Bluetooth) signal.

Typically the radio beacon is configured to transmit signals complyingwith a short range (i.e. typically a range of less than 500 m, moretypically a range of less than 200 m, even more typically a range ofless than 100 m) wireless connectivity standard, such as Wi-Fi orBluetooth. In one embodiment, the radio beacon may be a connectableradio beacon, such as a Wi-Fi access point. More typically, the radiobeacon is a Bluetooth beacon. That is, the radio beacon is typicallyconfigured to transmit signals complying with one of the Bluetooth shortrange wireless connectivity standards. The radio beacon may, forexample, be compatible with the Bluetooth short range wirelessconnectivity standard core specification version 4.0 or later. TheBluetooth short range wireless connectivity standard core specificationversion 4.0 is advantageous at is allows transmission at any of aplurality of discrete transmit power levels. Bluetooth is a trade markof the Bluetooth special interest group. The Bluetooth short rangewireless connectivity standard core specification is published from timeto time at the domain www.bluetooth.org.

The radio beacon may be operable to detect radio beacons (includingradio beacons according to the invention and other radio beacons whichare not according to the invention, for example, known wireless accesspoints) and to transmit data concerning detected radio beacons to a datacollection device, such as a server. The data which is transmittedtypically comprises identifiers of the radio beacons detected by theradio beacon (e.g. MAC addresses). The data which is transmittedtypically comprises signal strength data concerning the strength ofsignals received from detected radio beacons. The data may be sentthrough the internet or through a gateway device, for example apositioning controller which sends control signals to and receives datafrom the radio beacon, or a user device which is periodically deployedto retrieve the data from the radio beacons.

The invention also extends in a third aspect to a positioning controllercomprising at least one radio transceiver, programmed to communicatewith a user device and one or more radio beacons according to the secondaspect of the invention, and to transmit control signals to one or moreradio beacons to cause the radio beacons to change (typically reduce)their transmit power level to facilitate estimating the position of theuser device.

The controller may be programmed to receive signals to change (typicallyreduce) the transmit power of one or more radio beacons from a userdevice and to send signals to change (typically reduce) the transmitpower level of one or more radio beacons responsive thereto.

The controller may be programmed to determine when to issue signals tochange (typically reduce) the transmit power level of one or more radiobeacons and to issue said signals. The controller may be programmed todetermine when to issue said signals to facilitate the estimation of theposition of a plurality of user devices in communication with thepositioning controller at once.

The controller may be programmed to transmit data concerning the radiobeacons which it can detect to a data collection device, such as aserver. The transmitted data may, for example, comprise the identifiersof one or more radio beacons which it can detect. The transmitted datamay, for example, comprise the received signal strength of signals fromone or more radio beacons which it can detect.

Optional features described in relation to the first, second or thirdaspect of the invention are optional feature of each of the first,second and third aspects of the invention.

The invention also extends to a computer program which, when executed bya processor, causes the processor to estimate the position of a userdevice by the method of the first aspect of the invention. The computerprogram may be adapted to be executed by the processor of a user deviceto estimate the position of the user device. The computer program may beadapted to be executed by the positioning controller. The invention alsoextends to a computer program which, when executed by a processor of aradio beacon, causes the radio beacon to function as a radio beaconaccording to the second aspect of the invention.

The said computer programs may be stored on a tangible computer readablemedium, such as a memory (e.g. RAM, ROM, PROM, EPROM, EEPROM), oroptical or magnetic disk.

DESCRIPTION OF THE DRAWINGS

An example embodiment of the present invention will now be illustratedwith reference to the following Figures in which:

FIG. 1 is a schematic diagram of a beacon;

FIG. 2 is a schematic diagram of a system comprising a plurality ofnon-connectable beacons and a user device;

FIG. 3 is a schematic diagram of a system in which the radio beacons areconnectable beacons;

FIG. 4 is a schematic diagram of an alternative system including acontroller;

FIG. 5 is a schematic diagram of a controller for use in the embodimentof FIG. 4;

FIG. 6 illustrates the variation in the coverage of a beacon withtransmit power level;

FIG. 7 is a flow diagram of a method of estimating the position of auser device by measuring distance to one or more radio beacons;

FIG. 8 is a flow diagram describing the default operation cycle of abeacon; and

FIGS. 9A through 9D are a flow diagram describing a procedure forestimating position where the transmit power level of individual radiobeacons can be changed.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

With reference to FIG. 1, a radio beacon 1 has a system on chip IC 2(which may for example, be a CC2540 from Texas Instruments, CSR1000 orCSR 1001 from CSR, EM9301 from EM Microelectronics or nRF800 series fromNordic semiconductor which runs Bluetooth short range wirelessconnectivity standard version 4.0 protocol for a single mode device,including the Bluetooth 4.0 location and proximity profiles) including aprocessor 4, RAM memory 6, a Bluetooth short range wireless connectivitystandard version 4.0 protocol communications module 8 and a radiofrequency transceiver module 10. The memory stores program codeexecutable by the processor in use and data, including an ID (e.g. MACID) of the radio beacon, and the position of the radio beacon (forexample, as latitude, longitude and optionally altitude).

The radio frequency transceiver module 10 is in electrical communicationwith an antenna 12 and the processor is in electrical communication witha sensor chip 14, which may for example be a temperature or pressuresensor for measuring ambient temperature or pressure. The beacon alsoincludes a battery 16 as power supply. The Bluetooth short rangewireless connectivity standard version 4.0 is useful for the method ofthe present invention as it allows for the transmission of radio signalsat any of a plurality of discrete transmit power levels. The transmitpower level of the radio beacon, from amongst the plurality of discretetransmit power levels, at which the radio beacons transmits at any giventime is selectable, e.g. by a processor. The available power levels willtypically depend on the specification of the particular system on achip. In an example embodiment, there are three power levels, −4 dBm, 0dBm and 4 dBm. The processor can instruct the Bluetooth short rangewireless connectivity standard version 4.0 protocol module to change thetransmit power level to between the available transmit power levelsunder the control of a program stored in the memory.

The beacon may be a connectable beacon, to which other devices canconnect, or a non-connectable beacon. Non-connectable beacons accordingto the invention change their transmit power level according to aprogram. Connectable beacons can change their transmit power levelresponsive to an instruction from a user device or positioningcontroller. They may also change their transmit power level according toa program. In some embodiments, the positioning controller may beprovided as a cloud service, i.e. the positioning controller maycomprise a functional module (e.g. executable program code stored on atangible computer readable medium and executed on a microprocessor) of aserver which communicates with the Bluetooth beacons over the internet.

FIG. 2 illustrates a system comprising a plurality of non-connectablebeacons 1 and a user device in the form of a Bluetooth enabled cellulartelephone 18. FIG. 3 illustrates a corresponding system in which theradio beacons 1 are connectable beacons, which operate independently.

FIG. 4 illustrates an alternative embodiment in which a plurality ofconnectable radio beacons 1 are in bidirectional communication with acontroller 20 coordinates and controls the radio beacons. In thisembodiment, the user device is in bidirectional communication with thecontroller and receives signals from the radio beacons but need nottransmit signals to them (although bidirectional communication betweenradio beacons and the user device in this configuration is not ruledout). FIG. 5 is a schematic diagram of a suitable controller having aprocessor 22 in communication with a dual mode Bluetooth short rangewireless connectivity standard protocol version 4.0 controller 24 inturn in communication with an antenna 26. The controller has a powersupply 28 which may be a battery power supply or a circuit for receivingan external power supply.

With reference to FIG. 6, the distance at which a radio beacon isdetectable using a Bluetooth short range wireless connectivity standardversion 4.0 compatible device varies with the transmit power level. If aradio beacon has four transmit power levels a dBmW, b dBmW, c dBmW and ddBmW where a<b<c<d then the distance from the radio beacon at which itcould expect to be detected would be greatest for transmit power level dand successively less for transmit power levels c, b and a respectively.

During operation, the beacons broadcast their position and dataidentifying their output power level. Their position may be broadcast inthe form of latitude, longitude and altitude information, such as wouldtypically be used for a global satellite positioning service, or localcoordinates (x, y and typically also z) defined for a particularinstallation. The power level may be broadcast as a numerical valueindicative of transmit power level in suitable units, such as dBmW, oras a range at which the radio beacon would typically be detectable by acompatible device (e.g. a distance in meters or other units). If theuser device has an atypically good or bad ability to detect and receivedata from radio beacons it may take this into account in subsequentcalculations of position.

An example method of estimating the position of a user device isillustrated in FIG. 7. The procedure begins 50, whereupon the userdevice scans 52 for available beacons and retrieves data concerning thelocation and transmit power of those beacons, either from the beacons ifthey transmit that data, or from another source, for example byretrieving data from a database.

It is then determined 54 whether the user device has found fewer thanthree beacons. If at three or more radio beacons have been found it isdetermined 56 whether the number of beacons is exactly three, or greaterthan three. If three beacons are found, then the position of the userdevice can be determined 58 by trilateration.

As it receives data concerning the current output power level from eachbeacon, it can work out distance to each radio beacon despite the factthat the transmit power level of the radio beacon may be variable. Thereceived signal strength is compared with the current output power leveland the attenuation of the signal between the radio beacon and thereceiver can be used to estimate the distance to the radio beacon usingradio propagation models familiar to one skilled in the art. If morethan three radio beacons can be detected then typically the three forwhich there is the greatest received signal strength will be selected 60and used for trilateration. Nevertheless, other factors may be takeninto account, such as estimates of the accuracy of the position ofindividual radio beacons. If more than three radio beacons can bedetected then that provides additional information which can be used toimprove the position estimate.

If however, fewer than three beacons were found, the following proceduredepends whether only one beacon was detected 62. If only one beaconswere detected, then the distance from that beacon is calculated 64 usingthe location and transmit power data, received from the beacon or adatabase. The position of the user device can then be estimated 66 asbeing in a circle with radius equal to that distance, centered aroundthe known position of the beacon.

An estimate of distance from an individual beacon can be obtained usingknowledge of the transmit power (P_(t)) of the beacon and the receivedsignal strength in the form of received power (P_(r)), according to thefollowing formula:P _(r) =P _(t) G _(t) G _(r)(λ/4πd)²

Where G_(t) and G_(r) are transmit antenna and receive antenna gainsrespectively, λ is the wavelength of the relevant signal, and d is thedistance between the transmitter and the receiver. The transmit antennaand receive antenna gains can be taken into account in calculations,approximated to 1, or G_(t) could be included in the value of transmitpower level transmitted by the beacon or stored in a database inrelation to the beacon. Thus, the data concerning the transmit powerlevel may comprise P_(t), or P_(t) G_(t), for example. For beaconsaccording to the Bluetooth short range wireless connectivity standardversion 4.0 specification which operate at 2.4 GHz, λ=0.125 m.

Where position is calculated with reference to a single beacon, theposition of user device will be in the circle of distance calculatedusing the above formula, centered on the position of the beacon.

If, however two beacons have been found, then the distance to each oftwo beacons is calculated 68 and used to estimate the position of theuser device. The position of the user device can be determined as eitherof the two intersects between circles having a radius equal to themeasured distances from the two respective beacons, centered on the tworespective beacons. In some implementations, in that case, the positionof the user device is determined as being in a circle having a diameterequal to the distance between the two intersects, with a centre at themid-point between the two intersects. In some circumstances, ambiguityas to on which side of the line directly connecting two beacons the userdevice is located might be resolved using, for example, previousmeasurements of the position of the user device.

However, the invention also extends to embodiments in which the beaconschange their power level to facilitate positioning. As will bedescribed, it is also possible to carry out measurements of positionsimply by determining whether or not individual radio beacons can bedetected when they are transmitting at specific transmit power levels.

FIG. 8 is a flowchart of an operating procedure or a radio beacon which,in at least one operating mode, changes its power level periodically ina cycle. The process starts 100 when the beacon is switched on orinstructed to enter the default operating mode. It adopts a firsttransmit power level and broadcasts 102 data concerning its position andtransmit power level. After a period of time it determines whether it isbroadcasting 104 at the lowest (non-zero) output power level which isavailable to it. If not, it reduces 106 its power to the next loweroutput power level which is available to it and continues broadcastingdata concerning its position and transmit power level. If it is, then itincreases 108 its power to the highest output power level in a cycle ofa power levels. This need not be the highest transmit power level ofwhich the radio beacon is capable, to avoid unnecessary energyconsumption. The highest output power level in the cycle of power levelsmay be different for different radio beacons, for example, it may beselected depending on the density of other radio beacons suitable foruse in positioning. In an example embodiment, the beacon startsbroadcasting position and transmit power information at 4 dBmW, then itchecks if it is transmitting at lowest transmit power (that is −4 dBm).As it is not transmitting at the lowest possible power, the nextavailable lower transmit power (that is 0 dBm) is used to transmit thenext broadcast of data. Once the data is broadcasted, it checks if thetransmit power is lowest, as it is not the lowest the beacon sets it'stransmit power to the next lower transmit power (that is −4 dBm) andbroadcasts the data. Then the transmit power is checked again, as it isthe lowest transmit power available, the beacon now sets it transmitpower to the maximum transmit power available. Then the whole process isrepeated over and over.

It is then possible to estimate the position of the user device takinginto account whether or not individual radio beacons are detectable atparticular transmit power levels. If a radio beacon can be detected at atransmit power level at which it would typically be detectable at adistance of x meters, it can be deduced that the user device is locatedwithin x meters of the position of the radio beacon. If a radio beaconcannot be detected at a transmit power level at which it would typicallybe detectable at a distance of y meters, it can be deduced that the userdevice is located at more than y meters from the position of the radiobeacon. Thus, each detection of a radio beacon at a transmit power levelor the failure to detect a radio beacon at a transmit power levelprovides information as to a locus within which the user device islocated, or within which the user device is not located. Therefore, itis possible to obtain an estimate of the position of a user devicewithout, for example, calculations of distance from a radio beacon usingreceived signal strength. Thus, it is possible to consider simplywhether radio beacons can and cannot be detected at particular outputpower levels, without using techniques such as analysis of the numericalvalue of received signal strength to estimate distance along acontinuous scale. However, the method can be improved by adding analysesof a numeric value of received signal strength and estimating distanceto individual radio beacons.

One skilled in the art will appreciate that there are always someinaccuracies due to slight errors in the known position of a radiobeacon, measurement variations, effects due to environmental factors andso forth and so the resulting data may require processing, such asaveraging, or probabilistic calculations if it is not fully consistent.

In a further example embodiment, the user device (or positioningcontroller) can transmit a signal to a radio beacon to reduce its outputpower level to facilitate positioning. This allows the same calculationsto be made, but is quicker as there is no need to wait for the radiobeacon to reduce its output power level according to its existingprogramming.

In a further embodiment, the possibility of the user device (orpositioning controller) transmitting signals to radio beacons to reducetheir output level to facilitate positioning is combined with estimatesof distance from radio beacons of known position using techniques suchas analysis of received signal strength. FIGS. 9A through 9D illustratea procedure for estimating position using connectable radio beacons,which can receive instructions to reduce their output power level eitherdirectly from a user device, or from a controller. The procedure starts200 whereupon the user device scans 202 for detectable radio beacons. Ofthe radio beacons which are detected, the three with the highestreceived signal strength are selected 204 and connected to. Data fromthose three radio beacons (including at least an identifier (e.g. MACaddress), their current output power level and their position) is stored206.

At least the three selected radio beacons are then instructed 208 toreduce their output power level to the next lower level. It is thendetermined whether the connection has been lost with any of the threebeacons. If not, then it is determined 210 whether the three radiobeacons are transmitting at the lowest available output power level. Ifnot, then data from those three radio beacons is stored again and theprocedure repeats. If they are transmitting at the lower availableoutput power level, then the position of the user device is obtained bytrilateration 212 using the positions of the three beacons (and theoutput power information received from the radio beacons if necessary).For example, if the output power information received from each of thethree radio beacons is the same the centroid of the three beaconsposition may be used as an estimate of the position of the user device.

If on the other hand, the connection with any of the three beacons islost when the power is reduced, it is determined 214 whether theconnection has been lost with all three beacons. If so, then datapreviously saved concerning the position, received signal strength andtransmit power level of the three beacons is retrieved 216 and theposition of the user device is then estimated by trilateration 218 fromthe three beacons.

If, following the reduction of power, the user device is no longerconnected to all three radio beacons, it is then determined 220 whetherthe user device remains connected to beacons. If it does, it isdetermined 222 whether the two beacons are transmitting at the lowestavailable power level. If so, then because both beacons are transmittingat the lowest possible transmit power level, the beacon with the highestreceived signal strength is selected 224. The output power of thebeacons 2 to 8 is then set 226 to the maximum of the pre-definedtransmit power levels. The user device is then disconnected from thebeacons. The position is then calculated 228 using data from a singlebeacon.

If, on the other hand, it was found that the two beacons were nottransmitting at the lowest available output power level, then position,transmit power level and received signal strength data is stored 230 forthe radial beacon with the highest received signal strength. The tworadio beacons to which the user device is connected are then instructed232 to change their output power to the next lower level.

If at step 220, it was found that the user device is no longer connectedto as many as two radio beacons, it is then determined 234 whether theuser device is still connected to at least one beacon. If it is not,then the stored data concerning the beacon with the highest receivedsignal strength when two radio beacons were connected is retrieved 236and used to calculate 238 the position of the user device.

If the user device is connected to just one beacon, then it isdetermined 240 whether that radial beacon is transmitting at the lowestavailable transmit power level. If it is, then, since the beacon istransmitting at the lowest possible output power, then the availabledata (radio beacon position, radio beacon output power level andreceived signal strength) is used 242 to estimate the current positionof the user device. The transmit power of that radio beacon is then setto the maximum of the pre-defined transmit power levels and the userdevice and disconnects 244 from the radio beacon. Position of the userdevice is then estimated 246 using data from that single radio beacon

If, on the other hand, it was found that the single radial beacon wasnot transmitting at the lowest available transmit power level, then dataconcerning that radial beacon and the strength of the signal receivedfrom the radial beacon are saved 248, the radial beacon is instructed tochange its transmit power level to the next lower level 250, and it isreassessed 234 whether the user device remains connected to a beacon.

The radio beacons typically transmit information about their position,and their current output power level, as well as an identifier, such asa MAC address. They may transmit additional information, for example ameasurement of ambient temperature or pressure to facilitatepositioning. Such measurements facilitate positioning as they can be fedinto environmental models to enable more accurate calculations ofdistance from the attenuation of signals between a radio beacon and areceiver.

Separately, radio beacons may report data concerning the radio beaconswhich each can detect. For example, they may periodically transmit theMAC address and received signal strength of each radio beacon which theycan detect to a remote server.

In embodiments in which a controller is present, although the userdevice receives the data from the beacons, instead of the user devicerequesting the beacons to change the transmit power, the user device maysend this request to the controller and the controller passes on thisrequest to beacons. Alternatively, the data measured by the user deviceare passed to the controller which itself decides to change the transmitpower level of one or more beacons to facilitate determination of theposition of the user device. The use of a controller which controlsmultiple radio beacons is helpful to efficiently organise determinationof position in embodiments where there are multiple user devices in alimited volume, interacting with the same radio beacons. A controllermay also collect data, for example, a controller may periodicallytransmit data comprising the MAC address and received signal strength ofeach beacon which it can detect to a server. It may alternatively oradditional receive data from radio beacons comprising the MAC addressand received signal strength of each radio beacon which that respectiveradio beacon can detect, and transmit that data to a server.

A controller is also useful in embodiments in which a radio beacon isinstructed to reduce its transmit power level as, once the transmitpower level has been reduced, a user device may no longer be able todetect the radio beacon. A controller might calculate the position ofthe user device taking into account that a user device can no longerdetect a radio beacon, or transmit up to date information concerning thetransmit power level of a radio beacon which has reduced its powerlevel, to a user device. A user device may anyway be able to inferinformation concerning its position from determining that a radio beaconcan no longer be detected, using knowledge of its current transmit powerlevel e.g. because it is known to vary transmit power level according toa schedule (e.g. in a cycle) or from knowledge that it transmitted aninstruction to the radio beacon to reduce its transmit power level.However, in some further embodiments, a radio beacon may transmit asignal indicative that it is about to change (e.g. reduce) its transmitpower level and/or a signal indicative of the transmit power level towhich it is about to change, before changing its transmit power level.This can provide additional confidence that the reason that the radiobeacon can no longer be detected is due to the reduction in transmitpower level and not for some other reason, e.g. it having been switchedoff, having failed or having changed to another mode.

The implementations described above typically use trilateration andnearest neighbour algorithms for positioning the user device. However,one skilled in the art will be aware of other algorithms such as then-nearest neighbour algorithm, weighted nearest neighbour algorithm orprobabilistic methods for tracking the user device.

Although in the embodiments described above, the position of the radiobeacons is obtained from the signal transmitted by the radio beacons,the position of the radio beacons may be obtained by another knownmethod, for example, from a database of radio beacons. This data may bedownloaded from a remote server when required. Similarly, although inthe embodiments described above, the instantaneous value of the outputpower level of the radio beacons is obtained from the signal transmittedby the radio beacons, in alternative embodiments, the schedule of outputpower level of radio beacons is predetermined and can be calculated orread from a database. This requires both the radio beacon and theprocessor which is calculating the position of the user device (whetherthat be the processor of the user device, or a remote processor) to haveaccurately synchronised clocks.

Although in the embodiments described above the radio beacons changetheir power level in a cycle and can also reduce their power levelresponsive to a signal to facilitate position determination, in someembodiments, the radio beacons do not require to change their powerlevel in a cycle, simply to change their power level (typically reducetheir power level) responsive to a signal, to thereby facilitate powerdetermination.

Furthermore, although in the examples shown all of the radio beacons canchange their transmit power level, positioning may also take intoaccount measurements made following detection of other radio beaconswhich do not have the ability to change their transmit power level, forexample estimates of position may take into account measurements made ofthe distance from a user to wireless access points which transmit atonly a single transmit power level, using techniques known to the personskilled in the art.

Further variations and modifications may be made within the scope of theinvention herein disclosed.

The invention claimed is:
 1. A method of estimating the position of auser device comprising a Bluetooth receiver, the method comprising: theuser device detecting one or more Bluetooth beacons configured totransmit signals at any of a plurality of discrete transmit powerlevels, and a computer processing system comprising a processorcalculating an estimate of the position of the user device taking intoaccount transmit power data concerning the transmit power level of theone or more said Bluetooth beacons at the time when the respectiveBluetooth beacon is detected and beacon position data concerning theposition of the one or more said Bluetooth beacons, wherein for some orall of the Bluetooth beacons, the respective Bluetooth beaconautomatically switches between at least some of the plurality ofdiscrete transmit power levels in a cycle in which the transmit powerlevel decreases monotonically from a highest level through at least oneintermediate level to a lowest level before returning to the saidhighest level.
 2. A method according to claim 1, wherein the beaconposition data is received from the Bluetooth beacons.
 3. A methodaccording to claim 1, wherein the transmit power data is received fromthe Bluetooth beacons.
 4. A method according to claim 1, wherein, in atleast some circumstances, the method does not take into account anymeasure of the strength of the signals received from the one or moreBluetooth beacons beyond whether or not the Bluetooth beacons can bedetected.
 5. A method according to claim 1, comprising taking intoaccount that one or more Bluetooth beacons can not be detected at agiven point in time.
 6. A method according to claim 1, comprising takinginto account that a Bluetooth beacon can be detected at a time when itis outputting signals at a first transmit power level and cannot bedetected at a time when it is outputting signals at a second lowertransmit power level.
 7. A method according to claim 1, comprisingreceiving a measure of an environmental property which affects distancemeasurement with the radio signal received from the Bluetooth beacon. 8.A method according to claim 1, wherein at least some of the Bluetoothbeacons are Bluetooth beacons compatible with the Bluetooth short rangewireless connectivity standard core specification version 4.0 or later.9. A non-transitory computer readable medium storing a computer programwhich, when executed by the processor, causes the processor to estimatethe position of the user device by the method of claim
 1. 10. A methodof estimating the position of a user device comprising a Bluetoothreceiver, the method comprising: the user device detecting one or moreBluetooth beacons configured to transmits signals at any of a pluralityof discrete transmit power levels, a computer processing system,comprising a computer processor, calculating an estimate of the positionof the user device taking into account transmit power data concerningthe transmit power level of the one or more said Bluetooth beacons atthe time when the respective Bluetooth beacon is detected and beaconposition data concerning the position of the one or more said Bluetoothbeacons, and causing a Bluetooth beacon to change its transmit powerlevel to facilitate the estimation of the position of the user device,wherein the user device, or a positioning controller, is programmed togenerate a signal to cause a Bluetooth beacon to reduce its transmitpower from a first discrete transmit power level to a second lowerdiscrete transmit power level and wherein in response to receipt of asaid signal causing the Bluetooth beacon to reduce its transmit powerlevel, but before the Bluetooth beacon reduces its transmit power level,the Bluetooth beacon transmits a signal which is indicative that it isabout to change its power level and/or indicative of the transmit powerlevel to which it is about to change.
 11. A method according to claim10, wherein the transmit power level of the Bluetooth beacon isincreased to obtain a stronger signal.
 12. A method according to claim10, wherein the user device sends signals to the Bluetooth beacon tochange its transmit power level directly or indirectly.
 13. A Bluetoothbeacon comprising: a transmitter configured to transmit data using radiosignals at any of a plurality of discrete transmit power levels, aprocessing system including a computer processor, the processing systembeing programmed to change the transmit power level at which thetransmitter transmits to facilitate estimation of the position of a userdevice, wherein the Bluetooth beacon is configured to automaticallyswitch between at least some of the plurality of discrete transmit powerlevels, and wherein the Bluetooth beacon changes between discretetransmit power levels in a cycle, wherein the cycle is a cycle in whichthe transmit power level decreases monotonically from a highest levelthrough at least one intermediate level to a lowest level beforereturning to the said highest level.
 14. A non-transitory computerreadable medium storing a computer program according to claim to performthe method of claim
 10. 15. A non-transitory computer readable mediumstoring a computer program which, when executed by the processor of theBluetooth beacon, causes the Bluetooth beacon to function as a Bluetoothbeacon according to claim
 13. 16. A Bluetooth beacon according to claim13, wherein the Bluetooth beacon transmits its position in itstransmitted radio signal.
 17. A Bluetooth beacon according to claim 13,wherein the Bluetooth beacon transmits data concerning its currenttransmit power level in its transmitted radio signal.
 18. A Bluetoothbeacon according to claim 13, wherein the Bluetooth beacon transmitsdata to facilitate accurate distance measurement.
 19. A Bluetooth beaconaccording to claim 18, wherein the Bluetooth beacon comprises a sensorto measure a property of the environment around the Bluetooth beacon andis configured to transmit a measurement of that property in itstransmitted radio signal.
 20. A Bluetooth beacon according to claim 13,wherein the Bluetooth beacon is compatible with the Bluetooth shortrange wireless connectivity standard core specification version 4.0 orlater.
 21. A Bluetooth beacon according to claim 13, wherein theBluetooth beacon is configured to detect Bluetooth beacons and totransmit data concerning detected Bluetooth beacons to a data collectiondevice.
 22. A positioning controller comprising at least one Bluetoothtransceiver, programmed to communicate with a user device and one ormore Bluetooth beacons configured to transmit data using radio signalsat any of a plurality of discrete transmit power levels, wherein theBluetooth beacon is programmed to change the transmit power level atwhich it transmits to facilitate estimation of the position of a userdevice, and to transmit control signals to one or more Bluetooth beaconsto cause the Bluetooth beacons to change their transmit power level tofacilitate estimating the position of the user device.
 23. A positioningcontroller according to claim 22, wherein the controller is programmedto receive signals to change the transmit power of one or more Bluetoothbeacons from a user device and to send signals to change the transmitpower level of one or more Bluetooth beacons responsive thereto.
 24. Apositioning controller according to claim 22, wherein the controller isprogrammed to determine when to issue signals to change the transmitpower level of one or more Bluetooth beacons and to issue said signals.25. A positioning controller according to claim 22, wherein thecontroller is programmed to transmit data concerning the Bluetoothbeacons which it can detect to a data collection device.
 26. Anon-transitory computer readable medium storing a computer programwhich, when executed by a processor of a computing device having atleast one Bluetooth transceiver, causes the computing device to functionas a positioning controller according to claim
 22. 27. A Bluetoothbeacon comprising: a transmitter configured to transmit data using radiosignals at any of a plurality of discrete transmit power levels, and aprocessing system including a computer processor, the processing systembeing programmed to change the transmit power level at which thetransmitter transmits to facilitate estimation of the position of a userdevice, wherein the Bluetooth beacon is configured to change thetransmit power level at which the transmitter transmits responsive to areceived signal from a user device which is receiving data from theBluetooth beacon or a positioning controller, wherein the Bluetoothbeacon changes the transmit power level at which the transmittertransmits responsive to a received signal by reducing the transmit powerlevel at which the transmitter transmits from a first discrete transmitpower level to a second lower discrete transmit power level, and whereinthe Bluetooth beacon is configured in response to receipt of said signalcausing the Bluetooth beacon to reduce its transmit power level, butbefore the Bluetooth beacon reduces its transmit power level, totransmit a signal which is indicative that it is about to change itspower level and/or indicative of the transmit power level to which it isabout to change.